I presented a problem featured in my forthcoming book Thinking with Chess: Teaching Children Ages 5-14 (Mongoose Press, 2012). Here’s the problem, which I borrowed from Guliev, S. (2003). The manual of chess endings. Moscow, Russia: Russian Chess House: White: Kg6, Ne6, Nd6. Black: Kh8. What is the result with White to move? Now insert a black pawn on a6, with all other chessmen the same and White to move.
What is the result now? Denton High School students especially liked working on this problem.